Moistureproof microphone



Sept. 12, 1950 a... c. POCOCK MOISTUREPROOF MICROPHONE Filed Nov. 10. 1945 INVENTOR LYNDALL C. POCOCK ATTO R N EY Patented Sept. 12, 1950 MOISTUREPROOF MICROPHONE Lyndall Crossthwaite Pocock, London, England,

assignor, by mesne assignments, to Internationa1 Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application November 10, 1945, Serial No. 627,945 In Great Britain August 15, 1944 'Section 1, Public Law 690, August 8, 1946 Patent expires August 15, 1964 This invention relates to telephone transmitters or microphones of the variable-resistance type in which granules of conductive material, usually carbon, form one or more current paths between electrodes, and are arranged to be agitated in response to applied sound.

The invention is directed to providing microphones with protection against ingress of moisture.

The effect of moisture on carbon granules is to cause them to stick togethe and so to destroy the microphonic action necessary in a carbon transmitter.

It is diflicult but not impossible to seal the whole transmitter by using a metal case sealed to the diaphragm and sealed at the back electrode but when this is done the expansion of the trapped air, resulting from increase of temperature, produces sufiicient pressure to deform the diaphragm which is necessarily made of thin material.

It has previously been proposed in British Specification No. 522,226 to place carbon granules and electrodes in a sealed chamber without sealing the whole microphone so that expanding air in the microphone casing may escape Without causing pressure on the diaphragm. The total force on the flexible part of the sealed chamber is less than the force which would be produced on the diaphragm by the same pressure if the whole microphone were sealed due to the smaller size of the flexiblepart of the chamber, and the said flexible part can conveniently be made stiff enough to withstand the said force.

In the previous proposals however, protection was obtained at the expense of performance and it is the object of the present invention to construct a microphone in such a way that the conducting material is completely protected against moisture while transmission performance comparable to that of generally similar but unsealed transmitters is obtained over the range of temperature and pressure conditions experienced in service.

One feature of the invention comprises a microphone of the type described in which a movable electrode forms one wall of an otherwise rigidly constructed chamber which is effectively sealed against entry of moisture.

The sensitivity of a microphone in the lower part of its frequency range is approximately inversely proportional to the total stiffness of the moving system. Where, as in the present case, there is a need to concentrate stillness at one 2 Claims. (Cl. 179-122) point namely the electrode wall of the carbon chamber, the contribution to stiflnes's from other members of the structure must be suitably proportioned in order that a desired order of total stiffness may be obtained.

. A second feature of the invention comprises a microphone of the type described, in which the electrode chamber is completely sealed against entry of moisture and in which the space behind the diaphragm is ventilated, thereby avoiding the occurrence of excess pressure on the back of the diaphragm, and reducing considerably the contribution to total stiffness of the diaphragm in association with the air space behind it.

Analysis of the most severe requirements likely to be met in service in terms of the changes in pressure that will result in a closed chamber of approximately constant volume shows that transmitters should be able to operate with internal excess pressures up to 5 lbs/square inch when the temperature is normal (the pressure would be less when a lower temperature is simultaneously encountered). Further the transmitter must withstand under transport conditions internal excess pressures up to 12 lbs/square inch at normal temperature though at 0. this would be reduced to 9 lbs. per square inch. Other possible conditions to be met would impose an external pressure of up to 9 lbs/square inch on the electrode chamber.

It is evident that strong and permanent airtight sealing of the carbon chamber is necessary to meet the above conditions and that the flexibility necessary to allow movement of one electrode relative to the other must be achieved by means capable of supporting the specified static pressures without stressing the flexible member above its elastic limit whereby it would be per manently deformed.

Experience shows that seals which rely on organic materials in the class of Waxes have insuflicient strength and are liable to crack, while viscous liquids except in thin films are ultimately forced out when continued pressure is applied. The surfaces to be sealed can be forced into intimate contact by cold flowing material, such as lead, other soft metals, and soft alloys with a thin film of viscous liquid between them to complete the joint. A combination of lead and rubber seals may be used. 7

A third feature of the invention comprises a microphone of the type described, in which the joints between the several individual members forming the closed electrode chamber are protected from passage of moisture by seals of coldflowing material with thin films of viscous liquid or insertions of rubber instead of the viscous films.

According to a further feature of the invention in which the electrode chamber of a microphone is so constructed that the electrode system is effectively sealed against entry of moisture under conditions which may cause internal excess pressures of up to 9 lbs/square inch.

According to a subsidiary feature the electrode system is also completely sealed against entry of moisture such conditions which may cause external excess pressures of up to 9 lbs/square inch.

According to a further subsidiary feature the device will operate effectively under conditions which may cause internal excess pressures, of up to 5 lbs/square inch.

The invention will be clearly understood from the following description of certain embodiments with reference to the accompanying drawings in which:

Fig. 1 is a cross-sectional side elevation of a microphone;

Fig. 2 is a back view of the microphone shown in Fig. 1; and

Fig. 3 is a cross-sectional side elevation of the electrode chamber of a modified form of the microphone shown in Figs. 1 and 2.

Referring first to Figs. 1 and 2, the capsule casing I forms both the electrode and carbon granule chamber and also the support for the diaphragm Ill.

The back electrode 3 is mounted in an electrode holder 2 passing through an aperture in the casing I insulated by mica washers 4 and secured in position by metal washers 5 and a pressed lead seal 6.

A thin film of viscous material is applied between all jointing surfaces external to the casing 1.

The front electrode 1 is a flat metal circular disc formed with a central dome 8. The front electrode material could be any metal with a suitable surface; gold, platinum or rhodium plating would be satisfactory, or a carbonised surface as described in British Specification No. 526,212. It has been found, however, that carbonised nickel provides a suitable surface and that this material can be formed without damage to the carbon surface, so that the finished electrode is in a work hardened condition.

The front electrode is sealed into a seating in the casing I by a lead seal 9 engaging a groove in the seating, and a thin viscous film.

The diaphragm I is mounted on the casing I in known manner by the ring I2 and cover I3. The centre of the diaphragm is apertured and formed into a cylindrical or conical ring II with a serrated edge of a diameter such that it fits into the domed portion of the front electrode. The casing I is formed with holes N, Fig. 2, ventilating theback of the diaphragm.

A unit comprising an annular spring plate I4 and a wire gauze annulus I is mounted on the back of the casing I by a bayonet joint in known manner, grooves in the plate I4 engaging pips I9 on the casing. The plate I4 has apertures I6 opposite the holes I! in the casing.

When the microphone is placed in a known housing, spring contact fingers within the housing bear on the gauze I5: the gauze and plate are forced towards the back of the casing: The back of the diaphragm is still ventilated via holes I1, apertures I6 and the gauze, but the gauze prevents particles of foreign matter entering and wedging under the edge of the diaphragm.

The seals 6 and 9 are formed by pressure of the order of 4 to 5 tons per square inch at which pressure the lead flows and moulds the contacting surfaces intimately together.

The holes II in the casing I not only reduce air stiffness behind the diaphragm but also allow water to be easily shaken out if for instance the transmitter has been immersed in water.

The construction of the diaphragm and the method of assembling the diaphragm to the front electrode obviate the necessity for fine assembly limits and can also prevent increase in stilfness of the mechanical system arising from opposing stresses in the diaphragm and electrode.

After assembly the teeth II may be fixed to the front electrode dome 8 by sealing material to prevent easy passage of breath moisture through the joint into the transmitter case.

The diaphragm is of light metal enamel finished on both sides, the electrical circuit is not, in this transmitter, through the diaphragm itself.

The internal surface of the electrode chamber is enamelled, but the seating surface for the front electrode is kept free of enamel to form electrical contact between the front electrode and the casing which acts as one terminal, the other being the interior surface of the hole in the back electrode which is well plated internally for this purpose.

The lead seals and adjacent surfaces are painted externally with sealing compound to form a continuous film free from pin-holes.

In the modification shown in Fig. 3, the holder 2 for the back electrode is cut away at I8 to receive a. rubber sealing ring which will be substantially deformed on assembly. Rubber sealing could also be added for the front electrode. The rubber seal would obviate the necessity for viscous films between jointing surfaces.

In'Fig. 3 also, the centre portion ZI of the diaphragm I0 is not cylindrical or serrated, and it is in spring contact with the lip of the domed portion 8 of electrode 1.

Microphones according to the invention are capable of withstanding extremes of damp heat and dry heat, very low temperatures, conditions of condensation due to rapid change of air temperature, driving rain etc., immersion in water, and high altitudes, while retaining a high standard of efliciency, and freedom from carbon granule packing troubles.

In each embodiment, the additional stiffness of the mechanical system due to the method of assembling the diaphragm to the front electrode is offset by the elimination of air stiffness by ventilating the back of the diaphragm. The ventilation avoids the occurrence of excess pressure on the diaphragm, and reduces considerably the contribution to total stiffness, of the diaphragm in association with the air space behind it.

What is claimed is:

1. In an electro-acoustic transducer of the carbon granule type having a rigid housing including a back electrode for holding the granules closed by a flexible disc constituting the front electrode received within an annular axially extending shoulder formed integrally on said housin the improvement that comprises an annulus of coldflowing metal within said annular shoulder pressing against the outer surface of said front electrode and. engaging with an annular groove formed on the inner surface of said shoulder whereby the housing is eif ectively sealed against entry of moisture. I

2. A device as defined by claim 1 wherein the annulus of cold-flowing metal is formed of lead.

LYNDALL 'CROSSTHWAITE POCOCK.

REFERENCES CITED The following references are of record in the file of this patent: 1

UNITED STATES PATENTS Number Name Date.

1,141,107 Dean June 1, 1915 1,226,886

Holland May 22, 1917 Number Number Australia Apr. 4, 1939 

